Health Physics Pergamon Press 1977. Vol. 33 (September), pp. 325-331. Printed in Great Britain

ENVIRONMENTAL TRITIUM TRANSPORT FROM ATMOSPHERIC RELEASE O F MOLECULAR TRITIUM* C. E. MURPHY Jr., J. R. WATTS and J. C. COREY

Savannah fiver Laboratory, E.I. du Pont de Nemours & Co., Aiken, SC 29801 (Received 20 October 1976; accepted 24 February 1977)

Abstract-Some samples of soils, water, air, and vegetation collected along paths of two tritium releases from a tritium production facility at the Savannah River Plant, Aiken, South Carolina, showed measurable elevation in the levels of tritiated water. The highest concentrations of tritiated water were found in the soil, where molecular tritium had been converted to tritiated water. Patterns of tritiated water movement, after the initial exposure period, agreed with other investigators' observations of tritiated water transport in terrestrial ecosystems. Dispersion and uptake were measured for puffs released under different meteorological conditions. INTRODUCTION

ON TWO separate occasions, molecular tritium has been accidentally released to the atmosphere from a tritium production facility at the Savannah River Plant. The first release occurred at 0700 hr EST on 2 May 1974 and contained 479,000 Ci of tritium. The second release occurred at 2200hr EST on 31 December 1975 and contained 182,000 Ci of tritium. In both cases, more than 99% of the tritium was estimated to be in the molecular form. Dose calculations after the incidents indicated that an individual at the plant boundary could have received a maximum dose of 0.14mrem from the 2 May 1974 release and 0.014mrem from the 31 December 1975 release because of inhalation and skin absorption of tritiated water. Because of different meteorological and seasonal conditions during and immediately after the two release periods, the vegetative, soil, and atmospheric data collected after the releases presented a unique opportunity to

*The information contained in this article was developed during the course of work under Contract No. AT(07-2)-1 with the U.S. Energy Research and Development Administration.

analyze the effect of climatic variation on the dynamics of tritium transport in the terrestrial ecosystems along paths of the two releases. Most research on tritium movement in ecosystems has centered on the movement of tritiated water. Field experiments (Ko73) and corroborating simulations (Sa73) indicate that the soil acts as the most important longterm reservoir of tritiated water in terrestrial ecosystems. Corey and Horton (C068) showed that tritiated water in a water-saturated laboratory column of Savannah River Plant soil was displaced when they added water to the column. Thus, adding tritiated water to the soil surface results in a band of tritiated water which moves down o r up in the soil column in response to the soil water pressure gradient. Tritium is converted by living organisms into biochemical compounds (Ki71). These compounds are smaller but biologically important longterm reservoirs for tritium. The turnover time of tritium fixed in organic molecules will vary from a few minutes in materials like sugars, which are rapidly metabolized in respiration, to many years in relatively inert compounds like the cellulose of tree trunks.

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Uptake of tritiated water by plants can take place through roots (Ra65; Ko73; K170a). Tritiated water vapor can also diffuse into plants through the leaf stoma (Va63; K174b; Mu76b). In contrast to the extensive research with tritiated water and plants, the absorption of molecular tritium by ecosystems has received little attention. Cline (C153) showed that bean leaves contained elevated levels of tritiated water after exposure to molecular tritium. For more than 40 yr, microorganisms have also been known to reduce molecular hydrogen (Sc66). Murphy et al. (Mu76a) showed that soil microorganisms will absorb molecular tritium, and some of this tritium will be converted into tritiated water. The environmental survey after a release of 289,000 Ci of tritium gas, primarily molecular tritium from Lawrence Livermore Laboratory, showed measurable quantities of tritiated water in vegetation, water, and milk (My73). The highest concentrations of tritiated water were found in the vegetation. SAMPLE COLLECTION

To assess radiological health effects, environmental samples were collected along paths of the released puffs as soon as sample teams could assemble after each release incident. These paths were predicted by meteorological models with input data from the Savannah River Plant’s meteorological observation network (Cr74). Samples included the leaves and needles of trees, leaves of herbaceous vegetation, air moisture, crops, surface water, and soil water. Measurements on the samples have been compiled in two reports dealing with the environmental effects of the releases (Ma74; Ja76). In addition to samples collected to assess radiological health effects, samples of pine needles, herbaceous vegetation and soils within the Savannah River Plant boundaries were collected along transects perpendicular to the trajectory of the released material (Fig. 1). These samples were collected to follow the uptake and redistribution of tritium by ecosystems exposed to the released puffs. Needle and leaf samples collected from the lower branches of trees, from shrubs, and

FIG.1. Transects for environmental samples after the tritium releases of 2 May 1974, and 31 December 1975. The letters A and B delineate areas discussed in detail in the text.

from herbaceous vegetation were sealed in glass jars immediately after being picked. Surface soil samples were also sealed in glass jars. Deeper soil samples were collected by driving sampling tubes into the soil. After the first tritium release, plastic tubes used to collect surface soil samples were sectioned immediately and samples were sealed in glass jars. After the second release, deeper soil samples were collected by driving a metal, split-tube sampler into the soil. After each coring, the split tube was opened and the cores were sectioned in the field. The sections were then sealed in glass jars. SAMPLE ANALYSIS

Water, soil and vegetation samples were measured for tritium by the liquid scintillation counting methods developed by Butler (Bu6lj. Soil and vegetation samples were freeze-dried under vacuum to remove the water which was collected on a cold finger trap. The tritium in water samples was counted, without freeze drying, by the same methods.

ENVIRONMENTAL TRITIUM TRANSPORT RESULTS AND DISCUSSION

Atmospheric dispersion Figure 2 shows the concentration of tritiated water found in vegetation and surface soil collected after the 2 May 1974 release along a transect 16 km from the release site (Transect A, Fig. 1). Figure 3 shows the concentration of tritiated water found in samples collected along a transect 8 km from the source after the 31 December 1975 release (Transect B, Fig. 1). Two differences are immediately apparent when Figs. 2 and 3 are compared: for the December release, the

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concentration of tritiated water is much lower and the width of the transect affected by the puff is smaller. The difference between the widths of the two puffs agrees with meteorological predictions. The daytime May release took place when lower wind speeds (about 2 mlsec) and an unstable atmospheric condition (classified as a Pasquill C stability) were present. The nighttime December release took place when the wind speed was 10mlsec and a stable atmospheric condition existed (classified as a Pasquill D for spread calculations). The

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Distance ( k m )

FIG.2. HTO concentrations found in surface soil and vegetation after the 2 May 1974 release along Transect A, Fig. 1. 200

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b

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Pine Needles (January 2 , 1976)

(3

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0

0.2

,

,

0.4

0.6

0.8 Distance

1

1.2

1.4

1.6

(km)

FIG.3. HTO concentrations found in surface soil and vegetation after the 31 December 1975 release along Transect B, Fig. 1.

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combination of lower turbulent diffusion of the puff and less time for spread at any distance from the source resulted in a narrower path for the December release. Based on meteorological conditions at the time of the two releases, the estimated centerline concentrations of molecular tritium at Transects A and B were approximately equal to 100 pCi/m3. The fraction of tritiated water was estimated to be 0.6% in both cases, or 0.6 pCi/m3. Estimated centerline concentrations of tritium are approximately equal because the smaller amount of lateral mixing of the tritium from the December release balances the larger amount of tritium from the May release. Therefore, the higher concentration of tritium found in vegetation and surface soil after the May release is the result of the longer transit time of the puff because of the slower wind speeds, the larger puff size and the higher biological activity of the vegetation during the warmer, daytime conditions. The transit time of the May puff at the transect is estimated to have been 31 min. The transit time of the December puff at the transect is estimated to have been 6 min. Tritium uptake and conversion to HTO in vegetation and soil The behavior of the tritium movement in the forest ecosystem exposed by the release can be shown by the time series of data collected at selected locations on the transect sampled after the May release (Fig. 4). Tri2000,

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tium entered the free water of the ecosystem components through two modes: direct exchange of tritiated water for normal plant and soil water or absorption of molecular tritium and conversion to tritiated water. The most obvious mode is through direct exchange of tritiated water with normal water in the plants and soil. The moisture content of air was about 16glm' on 2 May 1974; therefore, the highest concentrations of tritium found in the surface soil and vegetation (Table 1) had not obtained equilibrium with the estimated centerline values. This information indicates that the tritiated water exchange between plants and soil and the tritiated water vapor fraction of the puffs were sufficient to explain the levels of tritiated water found in the plants and surface soil. However, Cline (C153) showed that when plants were exposed to molecular tritium gas, some molecular tritium was converted to tritiated water. Murphy et al. (Mu76a) also showed that molecular tritium was converted to tritiated water in soils and plants and the highest concentrations of tritiated water were usually found in the soil. Because the highest tritium concentrations were found in the soil, conversion of molecular tritium to tritiated water seems to be an important source of the tritiated water added to the ecosystems after the released puff had passed. If the tritium concentrations in water removed from the vegetation and soil were assumed to be the result of diffusion and exchange of tritiated water vapor from the released puffs, the tritium concentration in the foliage of the vegetation would be as high or higher than that in the soil because of the high surface area-to-volume ratio and the closer proximity of the vegetation to the puff. Table 1. HTO Concentrations at Transecfs A and 3,

pCilmlH,O 2 May 1974 31 December 1975

Days after Release

FIG. 4. HTO concentration found in vegetation during a 54-day period after the 2 May 1974 tritium release at one location on Transect A, Fig. 1.

Estimated centerline HTO concentration in air of released puff Highest measured HTO concentrations In air after passage of puff In pine needles In herbaceous vegetation In surface soil

3.8 x la'

3.8 x la'

618 135

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ENVIRONMENTAL TRITIUM TRANSPORT

However, if the conversion rate of molecular tritium to tritiated water was higher in the soil than in the vegetation, as indicated by the results of Murphy et al. (Mu76a), then the higher tritiated water concentrations in the soil would be attributed to the greater conversion of molecular tritium in the soil.

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The concentration of tritiated water in the herbaceous vegetation (primarily grass) is high during the puff passage (before the first measurement) and then decreases with time. The data for the herbaceous vegetation can be fit by equation (1) where concentrations are normalized by the concentration on the first day (ClC,);intercepts are CA= 0.26 and Tritium cycling in the vegetation and soil CB = 5.02. The exponent constants are A A = The initial distribution of the tritium in 4.80 and AB = 0.18, corresponding to integral ecosystems after the release distinguishes half-times of 0.144 and 3.48 days. behavior of the exposure to molecular tritium The difference between the response of the gas from those of the exposure to tritiated trees and that of the herbaceous vegetation water. After the puff of molecular tritium has can be explained on the basis that the highest passed, behavior of tritium in ecosystems is concentrations of tritiated water exist in the associated with the behavior of tritiated surface soil after the initial exposure. Bewater and tritiated organic compounds in the cause the herbaceous vegetation has a small ecosystems. Figure 4 shows the change in storage volume of water and shallow roots, concentration of tritiated water in the ve- the plants respond quickly to the high trigetation as a function of time after the May tiated water concentration of the surface soil 1974 exposure. Trees showed a pattern of to satisfy the transpiration demand of the increasing tritiated water concentration atmosphere. Trees have a much larger reaching a maximum sometime between the storage volume of water and the lag between 2nd and 8th days. After the 8th day, the uptake from the soil and appearance in the tritiated water concentration declined until leaves is much longer. During the period the 62nd day, when the tritiated water between initial tritium exposure and arrival of concentration approached local background. tritiated soil water in the tree roots and trunk, The falling section of the curve can be fitted tritiated water will move from soil to tree by a two-component exponential curve leaves by vapor flow to the canopy air space when the canopy is low enough to allow an increase in the concentration of tritiated C -= cAe-AA1 + C, e-'EB'2 (1) water vapor. Conditions immediately followCD ing a release are an example of a period when where tritiated water vapor flow from the soil to the C = concentration of tritiated water in canopy takes place (see Table 1). The preceding analysis of the behavior of the vegetation, CD= highest observed concentration of tritiated water in the vegetation indicates that the soil water is the major source of tritiated tritiated water found at day D, CA,Cg = constants representing the value of water during the period measurements were ClCD extrapolated to time equal being taken. A further investigation was zero for each of two exponential made by taking soil cores on the 56th and 79th days after the May 1974 exposure period decay components, AA, AB = exponential decay constants cor- and on the 8th and 65th days after the responding to two decay com- December 1975 release (Fig. 5). Analysis of cores taken after the May 1974 release inponents. dicates that the tritiated water had been The intercepts are C A =9.12 and Cg = displaced downward from the surface by 0.191, and the exponential constants are A A = rain, but remained in the rooting zone. 0.28 and A g = 0.031. The exponents cor- Analysis of cores taken 8 days after the respond to integral half-times of 2.45 and 22.7 December 1975 release also show the tritiated water in the soil. The tritiated water was days.

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Atmosphere

_. E

-

2:

c a 0

5c

FIG.5. HTO concentration found in soil after 2

May 1974 and 31 December 1975 tritium releases. displaced from the soil surface by rainfall which occurred between the release and the time of measurement. Analysis of cores taken 65 days after the December release showed no tritiated water in the upper 122cm of soil. This absence of tritiated water is consistent with the idea that water will drain out of the soil profile during the wet, low-evapotranspirational winter period after the December release but not after the drier, high-evapotranspirational period after the May release. CONCLUSION

Figure 6 shows the paths of tritium movement during and immediately after the passage of the puff of tritium from an SRP production facility. The modes of entry into the ecosystem are through exchange of tritiated water with plant and soil water and the conversion of molecular tritium to tritiated water. On the basis of laboratory experiments (Mu76a) and the high tritiated water concentration in the soil immediately after the release, processes converting molecular tritium to tritiated water are particularly active in the soil. Tritiated water contents in the vegetation after the release are most closely related to the withdrawal of tritiated water from the soil by the roots. Some tritium is redistributed between the soil and the vegetation by diffusion of tritiated water vapor from the soil immediately after the exposure period. This diffusion decreases and finally stops when the tritiated water from the roots

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HTO

FIG. 6. Paths of molecular tritium and tritiated water movement after a release of molecular tritium.

reaches the needles, and the gradient between the soil and needles also decreases. After rainfall, tritiated water is displaced from the surface soil and moved deeper into the soil profile. During periods of high evapotranspiration, little drainage from the soil profile occurs; and the tritiated water is slowly depleted by root withdrawal. During periods of high rainfall and low evapotranspiration, the tritiated water is displaced below the rooting zone; and it is not accessible to vegetation. REFERENCES

Bu61 Butler F. E., 1%1, “Determination of Tritium in Water and Urine,” Analyt. Chem. 33, 409. C153 Cline J. F., 1953, “Absorption and Metabolism of Tritium Oxide and Tritium Gas by Bein Plants,” PI. Physiol. 28, 717. C068 Corey J. C. and Horton J. H., 1968, “Movement of Water Tagged with ’H,3H, and “0 through Acidic Kaolinitic Soil,” Proc. Soil. Sci. SOC.Am. 32, 471. Cr74 Crawford T. V., 1974, Progress Report Dose-to-Man Program FY 1973, USAEC Report DP-1341, Savannah River Laboratory, E. I. du Pont de Nemours and Co., Aiken, SC.

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Systems, USERDA Report DP-1422, Savannah Ja76 Jacobsen W. R., 1976, Environmental EffRiver Laboratory, E. I. du Pont de Nemours and ects of a Tritium Gas Release from the SavanCo., Aiken, SC. nah River Plant on December 31, 1975, USERDA Report DP-1415, Savannah River Mu76b Murphy C. E. Jr. and Corey J. C., 1976, “Absorption of Tritiated Water Vapor from the Laboratory, E. I. du Pont de Nemours and Co., Atmosphere by the Needles of Pine Trees”, Aiken, SC. presented at the 4th Nat. Symp. Radioecology, Ki71 Kirchmann R. J., Van Den Hoek J. and Corvallis, OR, May 12-14, 1975, in: Ecol. SOC. Tafontaine A., 1971, “Transfert et Incorporation Am. Special Publ. No. 1, Radioecology and du Tritium Dans les Constituants de L’Herbe et Energy Resources (Edited by Cushing C. E. Jr.), du Lait, en Conditions Naturelles,” Health Phys. pp. 108-1 12 (Stroudsberg, PA: Dowden, Hut21, 61. chinson & Ross). K170a Kline J. R., Martin J. R., Jordan C. F. and Koranda J. J., 1970, “Measurement of Trans- My73 Myers D. S., Tinney J. F. and Gudiksen P. H., 1973, “Health Physics Aspects of a Large piration in Tropical Trees with Tritiated Water,” Accidental Tritium Release,” in: Tritium (Edited Ecology 51, 1068. by Moghissi A. A. and Carter M. W.), pp. 611K174b Kline J. R. and Stewart M. L., 1974, 623 (Phoenix, AZ: Messenger Graphics). “Tritium Uptake and Loss in Grass Vegetation which has been exposed to an Atmospheric Ra65 Raney F. and Vaadia Y., 1%5, “Movement and Distribution of THO in Tissue Water and Source of Tritiated Water,” Health Phys. 26, Vapor Transpired by Shoots of Heiianthus and 567. Nicotiana,” PI. Physiol. 40,383. Ko73 Koranda J. J. and Martin J. R., 1973, “The Movement of Tritium in Ecological Systems,” Sa73 Sasscer D. S., Jordan C. F. and Kline J. R., 1973, “Dynamic Model of Water Movement in in: Tritium (Edited by Moghissi A. A. and CarSoil Under Various Climatological Conditions,” ter M. W.), pp. 430-455 (Phoenix, AZ: Mesin: Tritium (Edited by Moghissi A. A. and Carsenger Graphics). ter M. W.), pp. 485-495 (Phoenix, AZ: MesMa74 Marter W. L., 1974, Environmental Effects senger Graphics). of a Tritium Gas Release from the Savannah River Plant on May 2, 1974, USAEC Report Sc66 Schlegel H. G., 1%6, “Physiology and Biochemistry of Knallgasbacteria,” Adu. Comp. DP-1369, Savannah River Laborztory, E. I. du Physiol. Biochem. 2 , 185. Pont de Nemours and Co., Aiken, SC. Mu76a Murphy C. E. Jr., Boni A. L. and Tucker Va63 Vaadia Y. and Waisel Y., 1%3, “Water Absorption by the Aerial Organs of Plants,” S. P., 1976, Conversion of Gaseous Molecular Physiologa PI. 16, 44. Tritium to Tritiated Water in Biological